Monocrystalline silicon substrates are widely used today as substrates for semiconductors. However, their characteristics do not always make them suitable for recent trends toward higher withstand voltages and higher frequencies. Hence, although expensive, use is starting to be made of monocrystalline SiC and monocrystalline GaN substrates. For example, by employing semiconductor devices made with silicon carbide (SiC), which is a semiconductor material having a greater forbidden bandwidth than silicon (Si), to build power converters such as inverters and AC/DC converters, a reduction in power loss unattainable with semiconductor devices that use silicon has been achieved. Compared with earlier art, the use of SiC-based semiconductor devices reduces loss associated with power conversion and promotes a lighter weight, smaller size and high reliability in the power converter. Monocrystalline SiC substrates are also under investigation as starting materials for nanocarbon thin-films (including graphene) as a next-generation device material.
Typical methods for producing such monocrystalline SiC substrates and monocrystalline GaN substrates are as follows. (1) Monocrystalline SiC substrates are produced using an SiC sublimation process that grows seed crystals while sublimating SiC by subjecting a high-purity SiC powder to an elevated temperature of at least 2,000° C. (2) Monocrystalline GaN substrates are produced by a process that grows GaN seed crystals within high-temperature, high-pressure ammonia or by additionally inducing the heteroepitaxial growth of GaN on a sapphire or monocrystalline SiC substrate. However, because these production processes are carried out under extremely exacting conditions and are complicated, the substrate quality and yield are inevitably low, resulting in very high-cost substrates, which is an impediment to their commercialization and widespread use.
The substrate thickness at which device functions actually manifest on these substrates is in each case between 0.5 and 100 μm. The remaining thickness portion carries out primarily a mechanical holding/protective function during substrate handling; that is, it serves primarily as a handle member (substrate).
Substrates wherein a monocrystalline SiC layer, for which the degree of thickness that allows handling is relatively thin, is bonded to a polyerystalline SiC substrate with an intervening ceramic such as SiO2, Al2O3, Zr2O3, Si3N4 or AlN or an intervening metal such as silicon, titanium, nickel, copper, gold, silver, cobalt, zirconium, molybdenum or tin have been studied recently. However, when the intervening material for bonding the monocrystalline SiC layer and the polycrystalline SiC substrate is the former (a ceramic), the fact that this material is an insulator makes electrode production at the time of device fabrication difficult; when the intervening material is the latter (a metal), metallic impurities contaminate the device and tend to give rise to a deterioration in the device characteristics, which is impractical.
Various art for ameliorating these drawbacks has hitherto been described. For example, JP No. 5051962 (Patent Document 1) discloses a method which involves bonding together, at the silicon oxide faces, a source substrate which is a silicon oxide thin film-bearing monocrystalline SiC substrate that has been ion-implanted with hydrogen or the like with an intermediate support (handle substrate) of polycrystalline aluminum nitride having silicon oxide formed on the front side thereof, thereby transferring the monocrystalline SiC thin-film to the polycrystalline aluminum nitride (intermediate support), subsequently depositing thereon polycrystalline SiC and then placing the workpiece in a HF bath so as to dissolve and separate the silicon oxide faces. However, in general, because the silicon oxide faces are very closely and strongly bonded together, the HF does not readily penetrate over the entire surface of the silicon oxide faces, particularly the center portions thereof, as a result of which separation is not easy and takes an excessive amount of time, making for very poor productivity. Another problem is that, when fabricating a large-diameter SiC composite substrate using this invention, a large amount of warpage arises due to the difference between the coefficients of thermal expansion for the deposited layer of polycrystalline SiC and for the aluminum nitride (intermediate support).
JP-A 2015-15401 (Patent Document 2) discloses, for substrates whose surfaces are difficult to planarize, a method wherein a polycrystalline SiC-supporting substrate surface is amorphously modified, without oxide film formation, by means of a fast atomic beam and a monocrystalline SiC surface is also amorphously modified, following which both surfaces are brought into contact and thermal bonding is carried out, thereby stacking a monocrystalline SiC layer on a polycrystalline SiC-supporting substrate. However, in this method, the fast atomic beam alters not only the exfoliated interface of the monocrystalline SiC but also the crystal interior. As a result, this monocrystalline SiC, even with subsequent heat treatment, is not easily restored to good-quality monocrystalline SiC hence, when used in a device substrate, template or the like, obtaining a device having high characteristics or a good-quality SiC epitaxial film is difficult.
In addition to these drawbacks, in order to bond together the monocrystalline SiC with the polycrystalline SiC of the supporting substrate in the foregoing art, the bonding interface must have a smoothness corresponding to a surface roughness (arithmetic mean surface roughness Ra) of 1 nm or less. Yet, SiC is said to be the next most difficult-to-machine material after diamond. Even when a monocrystalline SiC surface is amorphously modified, subsequent smoothing processes such as grinding, polishing or chemical mechanical polishing (CMP) take an extremely long time, making higher costs unavoidable. Moreover, because polycrystalline materials have grain boundaries, carrying out fast atomic beam amorphization so as to achieve in-plane uniformity is difficult, leading to problems with the bonding strength and warpage, which has created major obstacles to commercialization.